1. Technical Field of the Invention
The present invention relates generally to an improvement on a small-sized desk-top centrifuge using an AC adaptor designed to output a dc constant voltage to an electric motor for rotating a rotor.
2. Background Art
Typical desk-top centrifuges designed for personal use have an encoder coupled to an output shaft of an electric motor for a rotor to measure the speed of the rotor for rotor speed control and door lock control in which a door of the centrifuge is locked during rotation of the rotor, while the lock is released when the speed of the rotor is decreased.
The installation of the encoder on the output shaft of the motor, however, results in an increase in overall height of the centrifuge, thereby causing the center of gravity of the centrifuge to be shifted upward. This will lead to instability of the rotor during rotation at high speeds.
In order to facilitate ease of operation, the desk-top centrifuges have a door switch which is turned on when the door is closed to start the rotor automatically. There is, however, a problem in that a failure of the door switch may cause the rotor to rotate unexpectedly although the door is opened.
Usually, a power supply such as an AC adaptor designed to output a constant voltage is used in the desk-top centrifuges which has a safety guard system for avoiding the overcurrent. In order to prevent a start current initiating the operation of the motor from causing the safety guard system to be actuated undesirably, a resistor may be coupled in series with the motor. The use of the resistor, however, may result in a lack of the start current required to actuate the motor, especially after the centrifuge is kept in a low-temperature condition, for example, within a refrigerator
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of invention to provide an improved structure of a centrifuge which allows the center of gravity to be kept at a low level and which is designed to improve the safety in the event of a failure of a system operation.
According to one aspect of the invention, there is provided a centrifuge which comprises: (a) a dc power supply; (b) a motor driving a rotor; (c) a drive switching element disposed between the dc power supply and the motor; and (d) a controlling circuit receiving a voltage of power regenerated by the motor to provide a pulse width-controlled on-off signal to the drive switching element to control speed of the motor. The controlling circuit increases an on-duration in which the on-off signal is at an on-level with time.
In the preferred mode of the invention, the controlling circuit produces a first pulse-width controlled signal whose pulse width increases with time and a second pulse-width controlled signal derived by feedback control to keep the speed of the motor constant and selects one of the first and second pulse-width controlled signals whose pulse width is smaller as the pulse-width controlled on-off signal.
The centrifuge further comprises a motor decelerating switching element connected in parallel to the motor. When it is required to stop the motor, the controlling circuit provides an on-signal to the motor decelerating switching element for a given period of time to decrease the speed of the motor, after which the controlling circuit provides a pulse width-controlled signal to the motor decelerating switching element to turn on and off the motor decelerating switching element cyclically until the voltage of power regenerated by the motor decreases to a given level.
The centrifuge further comprises a door, a movable door hook designed to lock and unlock the door selectively, a spring bringing the door hook into a door-locked state to lock the door, and a solenoid energized by the controlling circuit to attract the door hook against an elastic pressure of the spring to bring the door hook into a door-unlocked state to unlock the door. The controlling circuit provides the on-signal to the motor decelerating switching element for the given period of time to decrease the speed of the motor and provides the pulse width-controlled signal to the motor decelerating switching element to turn on and off the motor decelerating switching element cyclically until the voltage of power regenerated by the motor decreases to the given level, after which the controlling circuit energizes the solenoid.
The centrifuge further comprises an interlock spring which holds the door hook being in the door-unlocked state and releases holding of the door hook when the door is closed, a door lock switch disposed between the dc power supply and the motor, a door switch disposed between the dc power supply and the motor in series with the door lock switch. The controlling circuit controls the door lock switch and the door switch to establish electric communication between the dc power supply and the motor after the door is closed, and the door hook is placed in the door-locked state.